Domestic Roof Drainage Systems

An important consideration when designing a roof drainage system is to ensure that appropriate overflow measures are put in place in order to prevent water from flowing back into the building. This document has been developed to assist Queensland Sheet Metal customers with designing their roof drainage systems to ensure that they comply with the current Building Code of Australia (BCA).

With a wide range of roofing supplies, rainwater systems, downpipes and guttering supplies available, we can help you with all aspects of your overflow systems.

The National Construction Code 2016 (NCC2016) – New Overflow Requirements

The NCC2016 Volume 2 contains new overflow requirements for domestic eaves gutters. Performance requirements can be met by designing and installing gutters and downpipes for a Class 1 (eg. domestic residence) or Class 10 (eg. open carport) in accordance with acceptable construction practice described in section 3.5.2 of the NCC2016, or by designing in accordance with AS/NZS3500.3 or AS/NZS3500.5, or a Performance Solution as described in the NCC.

Gutter Overflow Options

There are two options when designing your overflow systems:

1. Continuous system – this operates over the entire length of the gutter (eg. slotted gutter and/or controlled back gap using spacers).

2. Dedicated system – this collects and removes the overflow in a given location (eg. using a rainwater head).

A roof drainage designer can meet the overflow volumes by using either or a combination of the two options.

Responsibilities of the Designer

It is the designer’s responsibility to design a rainwater system which allows adequate drainage to occur. The designer may be an architect, a builder, a hydraulic engineer, a home owner, or a roofing or guttering contractor.

The design and installation of a rainwater system needs to comply with the NCC2016 requirements or the Building Code of Australia and Australian Standards AS/NZS 3500.3 Plumbing and Drainage, Stormwater Drainage or AS/NZS 3500.5 Plumbing and Drainage, Housing Installations.

Responsibilities of the Installer

It is the installer’s responsibility to ensure the rainwater system is installed as per the requirements outlined by the rainwater system’s designer.

Adequate fall towards the downpipes must be given to gutters (a minimum of 1 in 500 for eaves gutters and 1 in 200 for internal gutters).

The installer must ensure that the correct number of downpipes of sufficent size are installed, that they are clear of debris and able to discharge correctly.


To ensure longevity of your rainwater system and trouble-free performance, it is essential that regular maintenance takes place. Gutters must be regularly cleaned to prevent the build up of debris, fungus or any other materials that could block the flow of water from the roof.

How Queensland Sheet Metal Can Help

Queensland Sheet Metal is an industry leading provider of metal roofing supplies and rainwater systems, including guttering and downpipes. Our experts are highly experienced in these systems and can assist with appropriate product selection, compliance requirements, and practical roofing solutions. Call Queensland Sheet Metal on 07 3267 1010 or contact us online for fast, friendly service and all the help you need.

Information extracted from the NCC2016 Volume 2 and is relevant for Queensland.

Table 1

Rainfall Duration Intensities for Queensland
Locality 5 minute duration rainfall intensity (mm/h)
Average recurrence interval (ARI), once in
20 years 100 years
Bamaga 252 298
Brisbane 234 305
Ipswich 211 278
Victoria Point 245 320
Bundaberg 265 340
Cairns 229 278
Cloncurry 218 278
Innisfail 248 301
Mackay 250 316
Mt Isa 199 260
Noosa Heads 258 331
Rockhampton 229 300
Toowoomba 203 268
Townsville 235 300
Weipa 239 283


Table 2

Overflow Volume for Continuous Measure (L/s/m)
Design 5 minute duration rainfall intensity (m/h)